Sea ice melting
near Greenland presents visible evidence of a changing climate.
A new report by the U.N.-sanctioned Intergovernmental Panel on Climate
Change states that fossil fuel burning is "very likely"
to blame for observed changes in climate, such as decreasing sea ice
and rising ocean temperatures. Image is courtesy of Richard Alley.

The world is warming, and the burning of fossil fuels is "very likely"
to blame, according to a new report released today in Paris by the Intergovernmental
Panel on Climate Change (IPCC). IPCC is a nonpartisan group of thousands
of scientists from 180 governments that operates under the auspices of
the United Nations Environment Programme and the World Meteorological
Organization. The report is "a very emphatic reaffirmation"
of the seriousness of human-caused global warming, Richard Somerville
of the Scripps Institution of Oceanography said today at a press conference
to announce the release of the report.

The IPCC report, the fourth in a series of assessments
since the panel was established in 1988 to outline a scientific consensus
on current climate change and humans' role in it, is the firmest yet in
establishing a link between human activities and global climate change.
While the previous 2001 report stated that humans were "likely"
the cause of a global warming trend, the new report changes that language
to "very likely." The new term is not just semantics, but indicates
a quantifiable upgrade in scientific certainty, from 67 percent to "more
than 90 percent," said Somerville, who was lead author on the first,
overview chapter of the report.

In addition to updated model projections of future climate changes, the
new assessment includes observations of currently changing climate, such
as rising ocean temperatures, sea ice melting and retreating glaciers
 including data from the last six years, which are among the seven
warmest years on record. "To me, a highlight of the report is the
statement that 'warming of the climate is unequivocal,'" Kevin Trenberth
of the National Center for Atmospheric Research (NCAR) and lead author
of the report's chapter on observations of climate change in the atmosphere
and at Earth's surface, told reporters today.

The observation data are also largely consistent with earlier model projections,
and lend confidence to the new report's projections for future climate
change, Trenberth said. Globally, the models predict more intense and
longer heat waves in summer, shorter winters and more intense hurricanes
(see Geotimes, December 2006).

The observational data, as well as more sophisticated modeling techniques,
have improved the confidence of scientists in the results of the models,
and allowed them to make regional, smaller-scale predictions of climate
change over the next century for the first time as well, said Linda Mearns
of NCAR, who was lead author on the chapter describing model projections
of future climate change. Such changes in the United States include reduced
winter length throughout the country, increasing dryness in the Southwest
and increasing precipitation in the Northeast, where the additional water
vapor is also likely to increase the intensity of both storms and snowfall
in that region, she said.

Although sea-level rise is predicted, the report's models do not incorporate
some of the most recent, controversial scientific findings, including
how melting of the major ice sheets of Greenland and western Antarctica
might quickly and dramatically increase sea levels, prompting some scientists
to state that the report is ignoring the gorilla in the room. Trenberth
denied that the report's writers had overlooked this issue, however, saying
that there was simply not yet enough data to accurately quantify the impact
of the big ice sheets.

While the new report outlines what scientists currently know about global
climate change and includes some projections for the future, two other
IPCC working groups are currently completing additional reports on related
issues. One, which focuses on the vulnerability and impact of climate
change on more specific sectors, such as marine biological systems, coastal
regions and human health, will be released in early April, while a third
assessment, which looks at strategies for mitigating future climate change,
will be released in late April or early May.